A woman gassed her children to death in the family car so her ex-husband couldn't have custody of them on Christmas Day, a court has been told.

The Queensland Supreme Court today was told the mother, who cannot be identified, started planning their deaths in October 2002 after being issued with a Family Court order stating they would spend Christmas Day with their father.

One month later, on November 22, the bodies of her eight-year-old boy and 10-year-old girl were found in the garage of their home at Sandstone Point, near Bribie Island.

The Brisbane court was told the mother, 42, had given her children crushed-up sleeping tablets, before putting them in the back seat of the car, attaching a garden hose to the exhaust, and switching on the ignition.

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